Various means of chemical sensitization for increasing the light sensitivity of silver halide photographic emulsions are known. One typical method is sulfur sensitization, wherein the light sensitivity of a silver halide photographic emulsion is increased by adding thereto a very small amount of sulfur or a sulfur compound to form silver sulfide. Examples of such methods are shown in U.S. Pat. Nos. 2,410,689 and 3,501,313, West German Pat. No. 1,422,869, and Japanese Patent Publication No. 20533/74.
Other methods for increasing the light sensitivity of silver halide photographic emulsions are carried out by adding thereto a suitable reducing agent or gold compound. These methods are known as reduction sensitization and gold sensitization, respectively, and are disclosed in U.S. Pat. Nos. 2,399,083 and 3,297,446.
Furthermore, it is known to increase the light sensitivity of silver halide emulsions by a combination of these sensitization methods, as described in T. H. James, The Theory of the Photographic Process, 4th Edition, pp. 149-160 (Macmillan Pub. Co., 1977).
In these conventional sensitization methods, as the amount of sensitizer is increased to obtain higher light sensitivity, the silver halide photographic emulsions tend to form fog. It is difficult to control the formation of fog even by using antifoggants or stabilizers. Furthermore, the photographic characteristics of films prepared with silver halide photographic emulsions sensitized by such conventional sensitization methods change greatly when they are stored under high temperature and high humidity conditions.
Attempts to improve the light sensitivity of silver halide emulsions by prolonging the chemical ripening period of increasing the ripening temperature have not been successful, and are accompanied by an increase in the formation of fog.